Just as you can set the file position of a stream with fseek, you
can set the file position of a descriptor with lseek. This
specifies the position in the file for the next read or
write operation. See File Positioning, for more information
on the file position and what it means.
To read the current file position value from a descriptor, use
lseek (desc, 0, SEEK_CUR).
— Function: off_t lseek (int filedes, off_t offset, int whence)
The lseek function is used to change the file position of the
file with descriptor filedes.
The whence argument specifies how the offset should be
interpreted, in the same way as for the fseek function, and it must
be one of the symbolic constants SEEK_SET, SEEK_CUR, or
SEEK_END.
SEEK_SET
Specifies that whence is a count of characters from the beginning
of the file.
SEEK_CUR
Specifies that whence is a count of characters from the current
file position. This count may be positive or negative.
SEEK_END
Specifies that whence is a count of characters from the end of
the file. A negative count specifies a position within the current
extent of the file; a positive count specifies a position past the
current end. If you set the position past the current end, and
actually write data, you will extend the file with zeros up to that
position.
The return value from lseek is normally the resulting file
position, measured in bytes from the beginning of the file.
You can use this feature together with SEEK_CUR to read the
current file position.
If you want to append to the file, setting the file position to the
current end of file with SEEK_END is not sufficient. Another
process may write more data after you seek but before you write,
extending the file so the position you write onto clobbers their data.
Instead, use the O_APPEND operating mode; see Operating Modes.
You can set the file position past the current end of the file. This
does not by itself make the file longer; lseek never changes the
file. But subsequent output at that position will extend the file.
Characters between the previous end of file and the new position are
filled with zeros. Extending the file in this way can create a
“hole”: the blocks of zeros are not actually allocated on disk, so the
file takes up less space than it appears to; it is then called a
“sparse file”.
If the file position cannot be changed, or the operation is in some way
invalid, lseek returns a value of -1. The following
errno error conditions are defined for this function:
EBADF
The filedes is not a valid file descriptor.
EINVAL
The whence argument value is not valid, or the resulting
file offset is not valid. A file offset is invalid.
ESPIPE
The filedes corresponds to an object that cannot be positioned,
such as a pipe, FIFO or terminal device. (POSIX.1 specifies this error
only for pipes and FIFOs, but in the GNU system, you always get
ESPIPE if the object is not seekable.)
When the source file is compiled with _FILE_OFFSET_BITS == 64 the
lseek function is in fact lseek64 and the type
off_t has 64 bits which makes it possible to handle files up to
2^63 bytes in length.
This function is a cancellation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time lseek is
called. If the thread gets canceled these resources stay allocated
until the program ends. To avoid this calls to lseek should be
protected using cancellation handlers.
The lseek function is the underlying primitive for the
fseek, fseeko, ftell, ftello and
rewind functions, which operate on streams instead of file
descriptors.
— Function: off64_t lseek64 (int filedes, off64_t offset, int whence)
This function is similar to the lseek function. The difference
is that the offset parameter is of type off64_t instead of
off_t which makes it possible on 32 bit machines to address
files larger than 2^31 bytes and up to 2^63 bytes. The
file descriptor filedes must be opened using open64 since
otherwise the large offsets possible with off64_t will lead to
errors with a descriptor in small file mode.
When the source file is compiled with _FILE_OFFSET_BITS == 64 on a
32 bits machine this function is actually available under the name
lseek and so transparently replaces the 32 bit interface.
You can have multiple descriptors for the same file if you open the file
more than once, or if you duplicate a descriptor with dup.
Descriptors that come from separate calls to open have independent
file positions; using lseek on one descriptor has no effect on the
other. For example,
{
int d1, d2;
char buf[4];
d1 = open ("foo", O_RDONLY);
d2 = open ("foo", O_RDONLY);
lseek (d1, 1024, SEEK_SET);
read (d2, buf, 4);
}
will read the first four characters of the file foo. (The
error-checking code necessary for a real program has been omitted here
for brevity.)
By contrast, descriptors made by duplication share a common file
position with the original descriptor that was duplicated. Anything
which alters the file position of one of the duplicates, including
reading or writing data, affects all of them alike. Thus, for example,
will read four characters starting with the 1024'th character of
foo, and then four more characters starting with the 1028'th
character.
— Data Type: off_t
This is an arithmetic data type used to represent file sizes.
In the GNU system, this is equivalent to fpos_t or long int.
If the source is compiled with _FILE_OFFSET_BITS == 64 this type
is transparently replaced by off64_t.
— Data Type: off64_t
This type is used similar to off_t. The difference is that even
on 32 bit machines, where the off_t type would have 32 bits,
off64_t has 64 bits and so is able to address files up to
2^63 bytes in length.
When compiling with _FILE_OFFSET_BITS == 64 this type is
available under the name off_t.
These aliases for the `SEEK_...' constants exist for the sake
of compatibility with older BSD systems. They are defined in two
different header files: fcntl.h and sys/file.h.
L_SET
An alias for SEEK_SET.
L_INCR
An alias for SEEK_CUR.
L_XTND
An alias for SEEK_END.
Published under the terms of the GNU General Public License
